The U.S. Pat. No. 4,857,081 to Taylor and assigned to the assignees of the present invention, issued Aug. 15, 1989, provides a process for separating hydrocarbons and halogenated hydrocarbons from water contaminated therewith. The process utilizes hollow fibers made from regenerated cuprammonium or viscose cellulose. The hollow fibers are tubular nonporous membranes.
Industrial uses of the patented invention sometimes combine modules containing the hollow fibers with carbon beds. There is a need in the industry to reduce the organic loading on the carbon bed that is inherent with the process.
More specifically, in certain waste treatments where it is desirable to remove water from hydrocarbon contamination, soluble hydrocarbons remain in the water permeating the hollow fiber membranes being used. For example, in certain industrial runs, BTEX (benzene-toluene-ethylbenzene-xylene) and O&G (oil and grease) levels are essentially non-detectable but total organic carbon (TOC) levels have been found to be above maximum allowed levels for environmental discharge. In other cases, water soluble organic substances appear to increase the passage rate of BTEX through the cellulose hollow fiber membranes. Post-treatment of such permeates with carbon beds is often unsatisfactory as a method of reducing organic content to environmentally acceptable levels.
Hyperfiltration of contaminated waters through reverse osmosis membranes has been considered as method of treatment. However, such membranes have often proven to be very susceptible to fouling by particulates. Furthermore, contact of such membranes with water-dispersed oil, grease and aromatic hydrocarbons generally leads to rapid failure of the hyperfiltration operability because of severe fouling of membrane surfaces or damage to the membranes. Aromatic hydrocarbons, for example, are known to soften or dissolve components of reverse osmosis membrane devices.
Most reverse osmosis membranes have also been observed to permeate significant amounts of low molecular weight organic compounds contained in a contaminated feedwater. This is even more so in the case of nanofiltration membranes, which are a class of "loose" reverse osmosis membranes characterized as being more permeable to salts and organic compounds than customary reverse osmosis membranes.
It has now been found that a combination of two membrane processes, involving in sequence a first membrane treatment of a hydrocarbon contaminated feedwater with a cellulose membrane followed by hyperfiltration of permeate from the cellulose membrane by means of a reverse osmosis or nanofiltration membrane, is surprisingly effective in producing a product water having a greatly reduced content of hydrocarbons and other organic compounds.
The present invention provides a dual membrane apparatus used for processing such turbid waters contaminated with hydrocarbons so as to produce a purified water environmentally suitable for discharge, the purified water containing acceptable levels of total organic carbons.